Seismic shuttle logging tools are generally known for use in performing vertical seismic profile (VSP) surveys from boreholes to evaluate the surrounding underground formations. An example of a multi-shuttle seismic array is illustrated in FIG. 1 of the drawings. As illustrated in FIG. 1, a multi-shuttle seismic array 100 may comprise a number of seismic logging tools linked together by means of a cable and logged through a borehole while seismic signals are generated at the surface. In the example of FIG. 1, the logging cable 102 (which may range in length, for example, from 4 to 46 km) connects to a logging head 104 which may include an Electrically Controlled Release Device (ECRD) and a Versatile Seismic Imager (VSI) cartridge 106. The multi-shuttle seismic array may also include a VSI interconnect cable 108, and a plurality of seismic logging shuttles 112 which may function as seismic receivers. At each measurement level, each shuttle is anchored to the wall of a borehole during a measurement operation by means of an anchoring arm. VSP survey techniques typically require multiple measurements to be taken at different levels or depths in a borehole. As a result, the tool anchoring mechanisms must be released to allow the tools to be repositioned. After the tools have been repositioned at the next measurement level, the tool anchoring mechanisms are then re-engaged before new measurements are performed.
FIG. 2 shows a detailed view of a conventional anchoring mechanism 200. The mechanism includes a permanent magnet (or electromagnet) brake 212, motor 214 and reducer arrangement 216 housed in the shuttle body 210. The output drive from the reducer 216 connects through a joint 218 and bearing 220 to a ball screw 222. The ball screw 222 drives a nut 224. The end of the screw 222 projects into the hollow end of a push rod 226 and the end of the nut 224 engages the outer surface of the push rod 226 through a clutch mechanism 228 which is described in more detail below. The end of the push rod 226 is connected to a link 230 through which it drives the anchoring arm of the shuttle (shown partially at 227).
The inner end of the push rod 226 is formed into a base section 232 which fits inside the nut 224. The base section 232 also has extensions outside the nut 224 to provide a connection to a potentiometer (or a linear variable differential transformer “LVDT”) 234 which acts as position sensor for the push rod 226 and so can be used for a caliper measurement in the borehole using the anchoring arm. A compression spring 236 is located around the motor/ball screw mechanism inside the shuttle body and acts on the base section 232 so as to normally urge the push rod 226 and hence the arm, outwards. The extension of the push rod 226 under by the spring 236 is limited by the position of the nut 224 on the screw 222 such that operating the motor 214 to move the nut 224 causes the push rod 226 to move out due to the spring 236 or be pulled in by the action of the nut 224.
Extension of the push rod 226 by the spring 236 is limited by either the arm contacting the borehole wall or by the base section 232 reaching the stops 238 positioned in the body (fully extended). Once the arm contacts the borehole wall, the nut 224 moves over the push rod 226 to activate the clutch 228 such that the screw 222 and nut 224 drives the push rod 226 directly and forces it against the borehole wall to anchor the shuttle. To release the arm, the motor is reversed and the screw 222 retracts the nut 224, releasing the clutch 228. The arm is then only held against the borehole wall by the spring 236 and so can move in or out as the shuttle is dragged up to a different position in the well. It is not necessary to retract the arm completely. If it is desired to retract the arm completely, the reverse motor drive is continued and the nut 224 is retracted along the screw 222 until it contacts the base section 232 of the push rod 226 which it then pulls back against the action of the spring 236 to retract the push rod 226 and thus the anchor arm. When the arm is fully retracted, the motor stalls and this is detected to find the fully retracted/closed position of the arm. The output from the potentiometer 234 can also be used to detect the arm in its fully retracted position. A detailed description of conventional anchoring mechanisms such as that illustrated in FIG. 2 is described in U.S. Pat. No. 6,315,075 to Nakajima, incorporated herein by reference in its entirety for all purposes.
Recently, the size of borehole seismic arrays has been increasing to more than 40 shuttles (at costs exceeding US$1,000,000). As a result, there is growing concern about losing assets (e.g., multi-shuttle seismic arrays) in a borehole. More importantly, the loss of a potentially important hole section and the cost of rig time may be an even greater concern if a seismic array retrieval operation (commonly referred to as a “fishing operation”) is ineffective.
For example, in one undesirable scenario, a multi-shuttle seismic array may become stuck in a borehole due to a failure in retracting the seismic shuttle anchoring arms. In order to handle such scenarios, a mechanical weakpoint is typically built into the anchoring arm linkage and designed to break under a certain pulling force. The breaking of the anchoring arm linkage allows the arm a few inches of travel so that the shuttle can be freed. However, this final, forceful freeing procedure may not be successful in cased holes where lack of friction does not allow sufficient force to be transferred to the anchor-arm weakpoints.
Accordingly, it will be appreciated that there exists a continual need to improve multi-shuttle seismic array designs in order to provide improved seismic logging capabilities, performance, and reliability.